1. Field of the Invention
The present invention relates to a dot matrix printer, and more particularly to a dot matrix printer in which the displacement of the printing head is performed by a stepping motor.
2. Description of the Prior Art
In the wire dot printers and thermal printers there are often employed printing heads having printing elements vertically aligned in a line.
FIG. 1 shows the structure of a conventional thermal printer having such vertically aligned heating elements, wherein a thermal head 1 is fixed on a carriage 3 slidably mounted on a horizontal slide member 2 and is positioned to be in contact with a platen 6 across a printing sheet 5 supplied from a paper roll 4.
Said carriage 3 is fixed by a mounting member 3a to an endless belt 9 provided between pulleys 7 and 8. Said pulley 8 is coaxially fixed to a pulley 10 of a larger diameter which is linked by a belt 14 to a pulley 13 fixed on an output shaft 12 of a DC motor 11. On said shaft 12 and under said pulley 13 there is fixed a slit disk 15 having radial slits 16 at determined angular intervals. Across said slit disk 15 there is provided a photocoupler 17 for detecting the rotational position of said output shaft 12.
FIG. 2 is a timing chart which indicates the heat pulses to be given to the thermal head 1 as the printing instrution signals are generated in synchronization with the output signals of said photocoupler 17.
The printer explained above is adequate for continuous carriage displacement in a line but is not suitable for intermittent carriage displacement because of the characteristic of DC motor. For this reason a stepping motor has been employed as the driving source for enabling both continuous and intermittent printing, and, in such case, the slit disk 15 and the photocoupler 17 can be dispensed with.
FIG. 3 shows the relationship between the excited phases of stepping motor, carriage movement and heat pulses wherein the carriage is displaced repeating vibration of a same pattern in response to each control pulse. Consequently the heat pulses can be generated in synchronization with the pulses supplied to the stepping motor.
The recent progress in the thermal head technology has enabled printing with pulses of a duration of 0.5 ms and an interval of 1 ms, which are ten times faster than the priority considered limits of pulse duration of 5 ms and pulse interval of 10 ms. In order to comply with such high printing speed, the stepping motor has to be driven with pulses of an interval of 1 ms or at a rate of 1000 pps. As the stepping motor provides a smaller output torque at a higher speed, there is therefore required an expensive stepping motor with an elevated power consumption for providing a higher torque and a higher frequency response.